352 research outputs found
Effects of electron-phonon coupling range on the polaron formation
The polaron features due to electron-phonon interactions with different
coupling ranges are investigated by adopting a variational approach. The
ground-state energy, the spectral weight, the average kinetic energy, the mean
number of phonons, and the electron-lattice correlation function are discussed
for the system with coupling to local and nearest neighbor lattice
displacements comparing the results with the long range case. For large values
of the coupling with nearest neighbor sites, most physical quantities show a
strong resemblance with those obtained for the long range electron-phonon
interaction. Moreover, for intermediate values of interaction strength, the
correlation function between electron and nearest neighbor lattice
displacements is characterized by an upturn as function of the electron-phonon
coupling constant.Comment: 5 pages and 4 figure
A cellular automaton for the factor of safety field in landslides modeling
Landslide inventories show that the statistical distribution of the area of
recorded events is well described by a power law over a range of decades. To
understand these distributions, we consider a cellular automaton to model a
time and position dependent factor of safety. The model is able to reproduce
the complex structure of landslide distribution, as experimentally reported. In
particular, we investigate the role of the rate of change of the system
dynamical variables, induced by an external drive, on landslide modeling and
its implications on hazard assessment. As the rate is increased, the model has
a crossover from a critical regime with power-laws to non power-law behaviors.
We suggest that the detection of patterns of correlated domains in monitored
regions can be crucial to identify the response of the system to perturbations,
i.e., for hazard assessment.Comment: 4 pages, 3 figure
Noise-assisted Thouless pump in elastically deformable molecular junctions
We study a Thouless pump realized with an elastically \textit{deformable
quantum dot} whose center of mass follows a non-linear stochastic dynamics. The
interplay of noise, non-linear effects, dissipation and interaction with an
external time-dependent driving on the pumped charge is fully analyzed. The
results show that the quantum pumping mechanism not only is not destroyed by
the force fluctuations, but it becomes stronger when the forcing signal
frequency is tuned close to the resonance of the vibrational mode. The
robustness of the quantum pump with temperature is also investigated and an
exponential decay of the pumped charge is found when the coupling to the
vibrational mode is present. Implications of our results for
nano-electromechanical systems are also discussed.Comment: 2 Appendices and figures adde
4e-condensation in a fully frustrated Josephson junction diamond chain
Fully frustrated one-dimensional diamond Josephson chains have been shown [B.
Dou\c{c}ot and J. Vidal, Phys. Rev. Lett. {\bf 88}, 227005 (2002)] to posses a
remarkable property: The superfluid phase occurs through the condensation of
pairs of Cooper pairs. By means of Monte Carlo simulations we analyze
quantitatively the Insulator to -Superfluid transition. We determine the
location of the critical point and discuss the behaviour of the phase-phase
correlators. For comparison we also present the case of a diamond chain at zero
and 1/3 frustration where the standard -condensation is observed.Comment: 5 pages, 7 figure
Phase transitions in the Potts spin glass model
We have studied the Potts spin glass with 2-state Ising spins and s-state
Potts variables using a cluster Monte Carlo dynamics. The model recovers the +-
J Ising spin glass (SG) for s=1 and exhibits for all s a SG transition at
T_{SG}(s) and a percolation transition at higher temperature T_p(s). We have
shown that for all values of at T_p(s) there is a thermodynamical
transition in the universality class of a ferromagnetic s-state Potts model.
The efficiency of the cluster dynamics is compared with that of standard spin
flip dynamics.Comment: 8 pages, Latex, with 8 EPS fig
Effects of the electron-phonon coupling near and within the insulating Mott phase
The role of the electron-phonon interaction in the Holstein-Hubbard model is
investigated in the metallic phase close to the Mott transition and in the
insulating Mott phase. The model is studied by means of a variational slave
boson technique. At half-filling, mean-field static quantities are in good
agreement with the results obtained by numerical techniques. By taking into
account gaussian fluctuations, an analytic expression of the spectral density
is derived in the Mott insulating phase showing that an increase of the
electron-phonon coupling leads to a sensitive reduction of the Mott gap through
a reduced effective repulsion. The relation of the results with recent
experimental observations in strongly correlated systems is discussed.Comment: 4 pages, 4 figure
Rashba effect induced localization in quantum networks
We study a quantum network extending in one-dimension (chain of square loops
connected at one vertex) made up of quantum wires with Rashba spin-orbit
coupling. We show that the Rashba effect may give rise to an electron
localization phenomenon similar to the one induced by magnetic field. This
localization effect can be attributed to the spin precession due to the Rashba
effect. We present results both for the spectral properties of the infinite
chain, and for linear transport through a finite-size chain connected to leads.
Furthermore, we study the effect of disorder on the transport properties of
this network.Comment: To appear in Phys. Rev. Let
Polaron features for long-range electron-phonon interaction
The polaron features for long-range electron-phonon interaction are
investigated by extending a variational approach previously proposed for the
study of systems with local coupling. The ground-state spectral weight, the
average kinetic energy, the mean number of phonons, and the electron-lattice
correlation function are discussed for a wide range of model parameters
focusing on the adiabatic regime and comparing the results with the short-range
case (Holstein model). A strong mixing of electronic and phononic degrees of
freedom for small values of the electron-phonon coupling constant is found in
the adiabatic case due to the long-range interaction. Finally a polaron "phase
diagram" is proposed.Comment: 4 figs., to appear in J. Phys.:Condens. Matte
Rashba effect in quantum networks
We present a formalism to study quantum networks made up by single-channel
quantum wires in the presence of Rashba spin-orbit coupling and magnetic field.
In particular, linear transport through one-dimensional and two-dimensional
finite-size networks is studied by means of the scattering formalism. In some
particular quantum networks, the action of the magnetic field or of the Rashba
spin-orbit coupling induces localization of the electron wave function. This
phenomenon, which relies on both the quantum-mechanical interference and the
geometry of the network, is manifested through the suppression of the
conductance for specific values of the spin-orbit-coupling strength or of the
magnetic field. Furthermore, the interplay of the Aharonov-Bohm phases and of
the non-Abelian phases introduced by spin-orbit coupling, is discussed in a
number of cases.Comment: 8 pages and 6 figure
Finite driving rate and anisotropy effects in landslide modeling
In order to characterize landslide frequency-size distributions and
individuate hazard scenarios and their possible precursors, we investigate a
cellular automaton where the effects of a finite driving rate and the
anisotropy are taken into account. The model is able to reproduce observed
features of landslide events, such as power-law distributions, as
experimentally reported. We analyze the key role of the driving rate and show
that, as it is increased, a crossover from power-law to non power-law behaviors
occurs. Finally, a systematic investigation of the model on varying its
anisotropy factors is performed and the full diagram of its dynamical behaviors
is presented.Comment: 8 pages, 9 figure
- …